Evaluation of uranium potential areas based on heliborne TDEM, magnetic and radiometric data interpretation in the western part of the Chhotanagpur Granite Gneissic Complex (CGGC), Central India

The E-W trending Chhotanagpur Granite Gneiss Complex (CGGC) of the Satpura Mobile belt is situated between Narmada-Son Lineament and the Central Indian Suture Zone. It exposes gneisses and younger granites, narrow belts of supracrustal enclaves, discontinuous boudin-type granulite and parallel shear zones. Field investigations have established uranium occurrence in shear zones, faults and fractures in this terrain apart from widespread shows of Nb–Ta and Rare Earth Element (REE) mineralisation. CGGC between Rasaganda and Kudri (N-S) tract indicates a major fold with limbs trending in ~ E-W direction and its closure lies near Ramanujganj. The fold patterns observed through satellite imagery clearly depict a NW plunging anticlinorium and younger granite plutons, viz., Dugru and Dubha emplaced within the core of the anticlinorium. Interpretation of high resolution heliborne geophysical data identified younger granite plutons in zones favourable for hosting granite-related uranium mineralisation in this terrain. TDEM response over the CGGC gneisses and migmatites show unique banding and folded patterns. EM response of these conductors in late channels suggest that they have depth persistence (upto ~400 m) and are steeply dipping. A few E-W trending discrete curvilinear conductors are seen creasing the afore mentioned granitoid plutons. Two E-W trending major magnetic linears cut across the migmatites around the granitoid plutons. Another set of three prominent ENE-WSW trending linears identified in magnetic data are inferred as shear zones which transect the Dubha Granitoid indicating that this feature is later to emplacement of the granitoid. Based on the crosscutting relation observed in the magnetic data, it is inferred that the E-W magnetic linears are post migmatisation and prior to granitoid emplacement, whereas the ENE-WSW trending inferred shear zones are later. The conductors show spatial correlation with major magnetic linears and at places, they transect them implying that they represent different features. The radiometric grid images show vivid patterns which were not known earlier. These patterns are likely to be due to mineralogical variation within the gneissic and magmatic terrain of CGGC. Curvilinear patterns observed in the radiometric ternary image representing the gneissic banding and folding / rheology of migmatites corroborates with signatures on magnetic images. Overlay analysis of the uranium occurrences over the inferred magnetic linear bodies and conductors suggests that the uranium mineralisation have an intimate relationship with the inferred shear zone and the peripheral parts of granite. (author)